1. Field of the Invention
The present invention relates to a reticle and an optical proximity correction (OPC) method, and more particularly, to an OPC method that uses a reticle incorporating an assist mask with a primary mask.
2. Description of the Prior Art
In semiconductor fabrications, quality of lithography process is critical to yield of semiconductor processes. In a lithography process, integrated circuit layout pattern disposed on a reticle (photo mask) is transferred into a photoresist layer coated on a material to be patterned. The patterned photoresist layer is then used as a hard mask to etch the material so as to form the integrated circuit layout in the material. By repeatedly patterning different material layers on a semiconductor wafer, integrated circuit is therefore fabricated.
In a lithography process, proximity effect has been observed. The proximity effect can be divided into two groups: optical proximity effect which depends on optical system behavior and non-optical proximity effect which results from some factors such as photoresist acid diffusion, flare, micro-loading effect, etc. In order to overcome the proximity effect, various OPC models have been proposed.
With reference to FIG. 1 and FIG. 2, FIG. 1 and FIG. 2 are schematic diagrams illustrating a conventional OPC method. As shown in FIG. 1, a reticle 10 has a pattern 12 to be transferred disposed thereon. The pattern 12 of the reticle 10 is identical to an integrated circuit layout to be transferred. Normally, the reticle 10 includes a dense region 14 in which the pattern 12 is arranged more densely and an isolated region 16 in which the pattern 12 is arranged less densely. This uneven distribution causes an uneven light transmittance in the exposure process, and therefore generates micro-loading effect. The micro-loading effect leads to pattern deviations. Specifically, the photoresist pattern may be different from the pattern 12 on the reticle 10. As shown in FIG. 2, the prior art OPC method forms a dummy pattern 18 on the reticle 10 to reduce the micro-loading effect. The dummy pattern 18 has a phase difference of 180 degrees from the pattern 12 so that the dummy pattern 18 is not transferred into the material to be patterned. However, the dummy pattern 18 is also formed in the reticle 10. This means the dummy pattern 18 cannot be changed after the reticle 10 is made. As long as any parameter of the lithography process needs to be changed or the dummy pattern 18 needs to be modified to optimize the quality of lithography process, the reticle 10 has to be redesigned. Since the manufacturing cost of a reticle is high, the conventional OPC method dramatically increases the cost of lithography process.